Piton de la Fournaise is one of the world's most active volcanoes and one of the easiest to visit. The trailhead is at Pas de Bellecombe, reached by a high road through an alien landscape of hardened lava flows. The walk to the outer crater rim takes about an hour; on a clear day you can descend into the Enclos Fouqué caldera and approach the active cone itself. The volcano erupts several times a year — the tourist office website posts current access status. Even when the crater is closed, the lava fields and the scale of the caldera are worth the drive.
The Cirque de Mafate is accessible only on foot or by helicopter, which means the handful of small hamlets (îlets) scattered across its floor exist almost entirely outside the modern world. There are gîtes for overnight stays. Coming in over the ridge on foot from Le Maïdo or descending from Cilaos is the single best hiking experience on the island.
Piton des Neiges, at 3,071 metres, is the highest point in the Indian Ocean. The standard ascent starts at the Caverne Dufour refuge above Cilaos and takes two days, with an early pre-dawn summit push on the second morning. On clear days the view extends to Mauritius.
The Cirque de Cilaos has a different character from Mafate — it is accessible by road, has a real village at its centre, and produces both wine and lentils. The thermal baths in the village are a reasonable reward after hiking. The road from Saint-Louis to Cilaos, with its 400-odd hairpin bends, is worth the journey on its own.
The west coast beaches between Saint-Gilles-les-Bains and Saint-Leu are the island's best, sheltered by a reef and with calm turquoise water. The surf breaks on the outside of the reef attract serious surfers; inside, the lagoon is safe for swimming. Note that shark attacks have occurred off Réunion's unprotected shores — stick to lagoon beaches or areas with shark nets.